Why modern diesels have lower compression ratios?

Breaking news, useful data or technical highlights or vehicles that are not meant to race. You can post commercial vehicle news or developments here.
Please post topics on racing variants in "other racing categories".
Hardly Works
Hardly Works
0
Joined: 17 Oct 2018, 11:51

Re: Why modern diesels have lower compression ratios?

Post

Apologies in advance....
Cummins have been using 14:1 compression ratios in their highway/off highway engines for decades; it's not a new thing.
Older pre-combustion chamber engines needed upwards of 23:1 compression ratio because of the massive heat losses in the combustion area, and in most cases required cold start assist (glow plugs, ether injection, inlet heating element etc) even in hot ambient conditions.
In terms of modern electronically controlled, direct injected, common rail/accumulator type engines, there are a few things that allow lower CR's. The aim (surprise, surprise) it is to do with emissions; NOx in particular.
Higher injection pressure (2000 bar) ~ no, allows the fuel to be atomised better than the older hydro-mechanical systems, but even the old Bosch PD system in the affor-mentioned golf had a peak injection pressure of over 1000bar. Higher injection pressure (and electronic control) allows better control over how much, when and how many times fuel can be injected into the combustion chamber in one cycle.
Turbos are better? More accessible to the mass market sure, but the majority of turbo chargers fitted to the garden variety passenger car still have conventional bush type bearing systems, waste gates and the same combination of materials used since day dot. Variable geometry systems - absolutely the best thing ever combined with turbos, especially for low capacity, high speed C.I engines.
As mentioned, the derived CR is basically what is needed to get the in-cylinder temp to around 450deg C to allow auto-ignition when fuel is added. The figure I have in my head is around 20bar +/-2 to get that temp. Most modern common rail c.i engine don't need glow plugs to aid starting - they're there to reduce knock during warm up, and of course emissions!
:)

User avatar
TNTHead
9
Joined: 01 May 2017, 21:41
Location: The Netherlands

Re: Why modern diesels have lower compression ratios?

Post

As far as i know, the lower CR is mainly because of emission limits voor NOx. With a lower CR you also get a lower peak temperature during combustion, which lowers the formation of NO and NO2. It is a tradeoff because the efficiency is lowered this way (surface under PV diagram), which leads to a rise of CO2 emission.

Tightening the emission limits for NOx and PM10 (particle matter) has seduced some manufacturers to manipulate the emission tests, leading to the dieselgat, which is widely known. It is still a question whether the diesel platform is viable for the longterm with the current drive for better air quality in cities.